88 



SPECIAL SENSES. 



drew back by little and little, keeping my Eight Eye fixt and 

 very steddy upon the same ; and being about 10. foot distant, 

 the second paper totally disappear'd." ] 



In this experiment, the rays of light from the paper 

 which has disappeared from view are received upon the punc- 

 tum caecum, at the point of entrance of the optic nerve. If 

 the observer withdraw himself still farther, the second circle 

 will reappear, as the rays are removed from the punctum 

 csecum. With the ophthalmoscope, the point of penetration 

 of the optic nerve may be readily seen in the living eye. If 

 the image of a flame be directed upon this point, the sensa- 

 tion of light is either not perceived, or it is very faint and 

 indefinite, and is then probably due to diffusion to other por- 

 tions of the retina. 



The relative sensibility of different portions of the retina 

 has been accurately measured by Yolkmann, and has been 

 found to be in an inverse ratio equal to about the square of 

 the distance from the axis of most perfect vision. This ob- 

 server calculated the distance between the sensitive elements 

 of the retina at which he supposed that two parallel lines 

 would appear as one. In the axis of vision, the distance was 

 0-00029", and, at a deviation inward of 8, it was 0-03186", a 

 diminution of acuteness of more than a hundred times. The 

 following table gives the results of these experiments : 



1 MARIOTTE, A New Discovery touching Vision. Philosophical Transactions, 

 London, 1668, vol. in., p. 668. 



